A Li—S battery generally comprises a lithium metal (Li(m)) anode, a cathode containing sulfur (S8) mixed with carbon (sulfur itself being a poor conductor), and a liquid electrolyte. During discharge, lithium from the anode is oxidized to form lithium ions and the sulfur is reduced in the cathode to generate Li-polysulfide species. The principle of charge-discharge reactions in a conventional liquid electrolyte is illustrated in FIG. 1.
The theoretical specific energy of lithium-sulfur (Li—S) batteries is about 3 to 5 fold higher (2567 Wh/kg) than that of Li-ion batteries. For this reason, and for its economic and environmental benefits, the Li—S battery technology is often referred to as one of the most promising Li-ion replacement technologies. However, several drawbacks have postponed its entry on the market, including poor cycle life, low cycle efficiency, severe self-discharge and questionable safety issues. This would be due to the Li-polysulfide species being, at least partly, soluble in the electrolyte, and, more fundamentally, to the insulating nature of sulfur and lithium sulfide limiting the utilization of this active material (see Zhang S. S. et al, 2013, J. Power. Sources, 231, pp 153-162).
Most efforts in improving the Li—S battery technology have been concentrated on modifications to the sulfur-containing composite (to trap the sulfur within the cathode, X. Ji et al, 2010, J. Mat. Chem., 20, pp 9821-9826). However, most of the proposed methods involve steps which are less applicable to a larger industrial production scale and/or involve higher production costs.
Some research groups have developed polymer electrolyte systems, for example using PEO homopolymers, in order to delay the solubility of polysulfide ions, but the cell performance showed instant degradations after the initial discharge (S. S. Jeong et al., 2007, Journal of Power Sources 174, pp 745-750).
An all-solid-state Li—S battery was described in Nagao et al, 2013, J. Power. Sources, 222, pp 237-242. This system includes a mesoporous cathode composite, a Li—Al alloy anode and a thio-LISICON (Li325Ge0.25P0.75S4) solid electrolyte. In spite of the extraordinarily high capacity, it shows low discharge voltage and limited power performance less than 0.1 C at room temperature.
There is a need for industrially applicable Li—S electrochemical cells, having at least one of: improved cycle life, better cycle efficiency, lower self-discharge, improved safety, and/or lower production costs when compared to other Li—S battery alternatives.